The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Known spark-ignition (SI) engines introduce an air-fuel mixture into each cylinder that is compressed in a compression stroke and ignited by a spark plug. Known compression-ignition (CI) engines inject pressurized fuel into a combustion cylinder near top dead center (TDC) of the compression stroke that ignites upon injection. Combustion for both SI engines and CI engines involves premixed or diffusion flames controlled by fluid mechanics.
SI engines may operate in different combustion modes, including a homogeneous SI combustion mode and a stratified-charge SI combustion mode. SI engines may be configured to operate in a homogeneous-charge compression-ignition (HCCI) combustion mode, also referred to as controlled auto-ignition combustion, under predetermined speed/load operating conditions. HCCI combustion is a distributed, flameless, kinetically-controlled auto-ignition combustion process with the engine operating at a dilute air-fuel mixture, i.e., lean of a stoichiometric air/fuel point, with relatively low peak combustion temperatures, resulting in low NOx emissions. An engine operating in the HCCI combustion mode has a cylinder charge that is preferably homogeneous in composition, temperature, and residual exhaust gases at intake valve closing time. The homogeneous air-fuel mixture minimizes occurrences of rich in-cylinder combustion zones that form particulate matter.
Engine airflow may be controlled by selectively adjusting position of the throttle valve and opening and closing of intake valves and exhaust valves. On engine systems so equipped, opening and closing of the intake valves and exhaust valves may be adjusted using a variable valve actuation system that includes variable cam phasing and a selectable multi-step valve lift, e.g., multiple-step cam lobes that provide two or more valve lift positions. In contrast to the throttle position change, the change in valve position of the multi-step valve lift mechanism is a discrete step change.
When an engine operates in a HCCI combustion mode, the engine operates at a lean or stoichiometric air/fuel ratio operation with the throttle wide open to minimize engine pumping losses. When the engine operates in the SI combustion mode, the engine operates at or near stoichiometric air/fuel ratio, with the throttle valve controlled over a range of positions from 0% to 100% of the wide-open position to control intake airflow to achieve the stoichiometric air/fuel ratio.
Combustion during engine operation in the HCCI combustion mode is affected by cylinder charge gas temperature before and during compression prior to ignition and by mixture composition of a cylinder charge. Known engines operating in auto-ignition combustion modes account for variations in ambient and engine operating conditions using calibration tables as part of an overall engine control scheme. Known HCCI engine control schemes include calibrations for controlling engine parameters using input parameters including, e.g., engine load, engine speed and engine coolant temperature. Cylinder charge gas temperatures may be affected by controlling hot gas residuals via engine valve overlap and controlling cold gas residuals via exhaust gas recirculation. Cylinder charge gas temperatures, pressure, composition may be influenced by engine environment factors, including, e.g., air temperature, humidity, altitude, and fuel parameters, e.g., RVP, energy content, and quality.
Combustion during engine operation in the HCCI combustion mode may be characterized in terms of combustion heat release, which may include combustion timing relative to piston position. Combustion timing may be described in terms of a mass-burn-fraction, which indicates a piston position at which a portion of the mass fraction of a cylinder charge is burned. A mass-burn-fraction of interest includes a CA50 point (in crank angle relative to TDC) at which an accumulated heat release reaches 50% of a total heat release of a cylinder charge.
The airflow control strategy changes in accordance with the combustion mode chosen, necessitating a change in position of slow and fast actuators in the system. An HCCI engine equipped with a cam phaser and a variable valve lift system, such as a two-step valve profile switcher, operates with the high-lift valve profile in SI mode and the low-lift valve profile in HCCI mode. This reduces pumping losses significantly in low and part load conditions. Because the engine is operated with wide-open throttle position in HCCI mode, cylinder air charge is controlled through cam phasing, by changing the intake and exhaust valve timings and thereby adjusting the effective cylinder volume at intake valve closing (IVC), while cylinder air charge is primarily controlled using the throttle to change intake manifold pressure in SI mode. Valve profiles and timings and intake manifold pressure associated with operation in the SI mode and the HCCI mode are substantially different for a constant engine load, increasing a likelihood of noticeable torque deviation and associated drivability penalties absent a suitable engine control strategy to mitigate such torque deviation.